1. Field of Invention
The present invention relates to the reduction of biomass production during activated sludge treatment of pulp and paper mill effluents. The present invention further relates to conditioning the activated sludge with an acid effluent.
2. Description of Prior Art
Biological sludge is an activated sludge produced by degradation and biosynthesis of dissolved organics, is known to be viscous and difficult to dewater. The present practice in mill sludge dewatering is to combine the biological or secondary sludge with the fibrous primary sludge, in order to improve dewatering rate and dryness of sludge cake. Decreases in primary sludge generation at the mill through improved reclamation of rejects, as well as the extensive use of activated sludge treatment systems in the industry, have resulted in an overall decrease in the weight proportion of primary to secondary sludge. This adversely affects efficiency of the dewatering process.
A pulp mill producing 750 tons of pulp per day will typically have a total effluent flow of 25 million US gallons/day which would produce 250,000 US gallons of sludge having a consistency of 1%, and requiring dewatering and disposal.
Sludge handling and disposal represent a substantial part of the activated sludge treatment costs. Approaches to reduce sludge production might include manipulation of the food to microorganisms (F/M) ratio and/or sludge retention time (SRT) in the biological system. For instance, pilot plant studies with bleached sulphite/groundwood effluent showed that about 60% of the BOD removed was converted to biomass when the activated sludge plant was operated at SRT between 4 and 5 days, as described by Lamorie, J. et al., "Activated Sludge Treatment of Market Sulphite and Newsprint Effluent at Stora Forest Industries Limited", Proceed. 1990 CPPA/TS Annual Meeting, Montreal, Quebec, January 1992. The subsequent adjustment to an extended aeration mode (SRT, 20 days) reduced biomass production by half (BOD conversion to sludge, 30%). However, in most cases the activated sludge plants are not designed to operate at high SRT. The disadvantages of such operation are low settleability of sludge and a potential for development of filamentous growth.
Degree of sludge biosynthesis can also be affected by the effluent composition. It was observed by Lee et al., Aerated Lagoon Treatment Upgrade, Proceed. 1985 TAPPI Environ. Conf., Mobile, Ala., April 1985, p. 97, that the biosynthesis coefficient decreased historically from 0.40 to 0.24 kg sludge/kg BOD removed, as a result of lesser availability of low-molecular weight carbohydrates due to an improved collection of black liquor at a bleached kraft mill.
Sludge lysis by mechanical means has been recently studied by Springer et al., Feasibility Study of Sludge Lysis and Recycle in the Activated Sludge Process, Proceed. 1993 TAPPI Environ. Conf., Boston, Mass., March 1993, p. 761. This approach is based on high-shear disintegration of waste activated sludge (WAS), in order to break the cell membranes and convert a part of the biomass into soluble substrate. The lysed sludge was to be returned into the aeration basin to redigest the released BOD components. When operated in extended aeration mode, the proposed process was expected to operate free of excess sludge. In terms of operating costs, Springer et al. estimated savings of about 30%, compared to a conventional activated sludge plant. A possible drawback of this process might be the formation of non-biodegradable cell debris which could be difficult to clarify in the subsequent biosynthesis stage. Consequently, elevated levels of COD in the treated effluent could occur.
A complete sludge digestion of WAS especially under alkaline conditions at elevated temperatures and pressures has been described by Lee, E. G., et al., Pulp Pap. Can., 77(6), 88 (1976). The process has yet not been used on a full scale.
Anoxic zones have been used as complementary treatment stages during the treatment of municipal wastewater. In practical terms this is achieved by splitting the aeration into two or more oxic stages, which allows for insertion of one or more anoxic i.e. non-aerated stages. For example, ammonia can be removed from municipal wastewater by a nitrification-denitrification processes which alternates oxic and anoxic treatment conditions as described in U.S. Pat. Nos. 4,173,531 and 5,137,636. Such treatments were also found effective at removing phosphate as described in U.S. Pat. No. 4,183,808. Phosphate can be also removed by air stripping of biological sludge in the activated sludge process as described in U.S. Pat. No. 4,956,094. The exposure of wastewater to anoxic and anaerobic conditions has been shown to enhance BOD and TSS removal in U.S. Pat. No. 5,128,040. UV irradiation has been proposed for sterilization of effluent streams in U.S. Pat. No. 5,174,898. However, the above uses of anoxic stages, as well as UV irradiation pertain to the improvement of municipal wastewater treatment efficiency, rather than to the reduction of sludge generation by the activated sludge process.
A process using intense aerobic/anaerobic digestion (HRT of 30 days, thermophilic temperatures) to hydrolyse a portion of waste primary and secondary sludge has been described in U.S. Pat. No. 4,915,840. This process aims to reduce the total mass of waste sludge by recycling the hydrolysed fraction of WAS back to the activated sludge process.
An exposure of the RAS to anaerobic environment for a period between 1-7 hours has been found to significantly reduce the WAS fraction in an activated sludge process in U.S. Pat. No. 3,235,487. The application of UV irradiation in a sludge biolysis unit to reduce the amount of biological sludge has been proposed in U.S. Pat. No. 3,591,491. A modification of the latter patent, using heat or vacuum instead of UV irradiation, has been described in U.S. Pat. No. 3,718,582.